Direct-positive silver halide emulsions containing boron hydrides

ABSTRACT

Direct-positive photographic elements having good sensitometric characteristics and aging stability are prepared by incorporating in the silver halide emulsion from 5 X 10 7 to 2.4 X 10 4 mole per 1.5 moles of silver halide of at least one boron hydride in which the skeletal framework forms a polyhedron or a fragment thereof, and which may contain heteroskeletal atoms selected from the groups C, S, and N, as a chemical fogging agent.

United States Patent Bigelow [151 3,637,392 [4 1 Jan. 25, 1972DIRECT-POSITIVE SILVER HALIDE EMULSIONS CONTAINING BORON HYDRIDESlnventor: John Howard Bigelow, Rochester, NY.

Assignee: E. I. du Pont de Nemours and Company,

Wilmington, Del.

Filed: Aug. 10, 1970 Appl. No.: 62,650

Related US. Application Data Continuation-impart of Ser. No. 8,417, Feb.3 1970.

US. Cl ..96/108, 96/109 Int. Cl. ..G03c l/28,G03c l/34 Field of Search..96/108, 109, 101

[56] References Cited UNITED STATES PATENTS 3,361,564 l/1968 Bigelow..96/101 3,445,235 5/1969 Burt ..96/101 Primary Examiner-Norman G.Torchin Assistant Examiner-Richard E. Fichter Attorney-Lynn BarrattMorris [5 7] ABSTRACT 13 Claims, N0 Drawings DIRECT-POSITIVE SILVERHALIDE EMULSIONS CONTAINING BORON HYDRIDES CROSS REFERENCE TO RELATEDAPPLICATIONS This application is a continuation-in-part of applicationSer. No. 8,417, filed Feb. 3, 1970, and is related to Assignees Burt,U.S. Ser. No. 736,276, filed June 12, 1968, and Burt & Ciavarri, U.S.Ser. No. 865,976, filed Oct. 13, 1969.

BACKGROUND OF THE INVENTION This invention relates to direct positivecolloid silver halide photographic emulsions. More particularly itrelates to such emulsions which are prefogged by a chemical foggingagent.

Direct positive emulsions are, of course, well known and there are manydifferent methods of producing direct positive images. For example, asilver halide emulsion may be given a short overall exposure of highintensity radiation and then given a longer imagewise exposure of lowerintensity. Upon development, a direct positive will be obtained. Anothermethod is to expose imagewise and develop and bleach out developedsilver and then flash expose and redevelop. A still further method is touse emulsion coated elements which have been chemically fogged with, forexample, formaldehyde, hydrazine, sodium arsenite, silver-ions and othernonsulfide fogging agents thus eliminating a solarizing exposure. Uponimagewise exposure and development of such chemically fogged elements, apositive image is obtained. In direct positive photographic elementsutilizing this latter method, there is usually incorporated adesensitizing compound, usually a colored desensitizing dye. Asignificant improvement was made in the art by the use of amine boranesas chemical fogging agents in direct positive emulsion systems. Directpositive emulsions and elements comprising such emulsions are describedand claimed in assignees Bigelow & Burt, U.S. Pat. Nos. 3,361,564 andBurt, U.S. Pat. Nos. 3,445,235 and assignees Overman, U.S. Ser. No.861,501, Sept. 26, 1969. The Burt patent discloses and claims the use ofrhodium and iridiurn salts in direct positive emulsions described in theabove Bigelow and Burt patent and the Overman Application disclose theuse of bismuth as antikink agents. The direct positive emulsions made asdescribed in the Bigelow & Burt and Burt patents are quite fast ascompared to direct positive elements of the prior art and suffer none ofthe disadvantages of long exposure times, low reversal densities,undesirable stain due to desensitizing dyes, as well as undesirablecontrast, and a multiplicity of exposure steps and processingoperations. Improvements in speed and other sensitometric properties ofthe direct positive layers of the above patents are taught by the aboveBurt and Burt and Ciavarri applications.

SUMMARY OF THE INVENTION It has now been found that the shelf life oraging stability and contrast of the direct positives disclosed in theabove patents and applications can be substantially improved by the useof a new class of borane compounds as chemical fogging agents. The newcompounds are boron hydrides in which the skeletal framework forms apolyhedron or fragment thereof and may contain heteroskeletal atomsselected from the group C, S, and N. They may be added to the emulsionsin amounts ranging from 5X10" to 2.4Xl" mole per 1.5 moles of silvernitrate. They may be added to the emulsion as a solution with a suitablesolvent, i.e., water, ethyl alcohol, benzene, acetone, dioxan, etc. Theuseful boron hydrides in aqueous solutions have greater stability thanthe amine boranes of the above patents. The effectiveness of some of theboron hydrides in fogging a silver halide emulsion is not affected bythe pH of the systems and they show good fogging properties under acidas well as alkaline conditions. Of course, certain of the boron hydridesare more effective than others when used in equivalent quantities. Thefogging amounts of particular boron hydrides within the above limits caneasily be deter mined empirically. The fogging effect produced by theboron hydride is bleachable by light exposure prior to development.

A suitable developer may be any conventional, alkaline, photographicdeveloping solution which would be used for standard processing ofdirect-positive emulsions in the absence of the boron hydrides. Althoughthe mechanism is not clearly understood, the light-bleaching effect isbelieved due to solarization. While the silver halide system used inthis invention is generally silver chlorobromide, other types, e.g.,silver chloride, silver iodobromide, silver iodochloride, and silveriodobromo chloride, may be used with the boron hydrides.

Among the boron hydrides which have been found useful as chemicalfogging agents in this invention are the boron hydrides of the series Bl-[ to 8 1-1 and the boron hydrides comprising the polyhedralazaboranes, polyhedral thiaboranes and polyhedral carboranes. Theseboranes and their preparation are described in: POLYHEDRAL BORANES byEarl L. Mueterties and Walter H. Knoth, published by Marcel Dekker,Inc., New York.

For most efficient fogging action it is preferred that the boronhydrides of this invention be added to the silver halide emulsion afterit has been made, ripened and washed to remove the excess soluble saltsresulting from the precipitation of the silver halides. Conveniently,the borane compounds are added just prior to or during the digestion orafter ripening period. The pH of the emulsion is adjusted to between 3.0and 9.0 and preferably between 5 and 8.5 and maintained at a preselectedlevel in this range during digestion. After digestion, the pH isadjusted to the desired coating pH and the conventional coating aids areadded. Coating aids such as saponin, sodium salts of polyethersulfonates, alkyl sulfonates, cetyl betaine, etc., can be utilized. Theemulsion is then coated and dried in the manner known to those skilledin photographic manufacturing methods.

The sensitometric characteristics of the direct positive emulsions maybe determined by exposing in an intensity scale sensitometer (describedon page 616, Mees, The Theory of the Photograph Process, MacMillanCompany, New York, 1942) using a suitable step wedge and light intensitydepending upon the inherent speed of the emulsion. Direct positiveemulsions are of course tailored to meet certain needs in the industryand are manufactured accordingly. After exposure in the sensitometer thestrips are developed in suitable developers as shown below, immersed ina shortstop bath and fixed for 3 minutes in a conventional fixer andthen washed and dried.

In evaluation of the processed strips, the toe is equal to the stepnumber at density 0.25 minus step number at density 0.01. The maximumdensity is the highest density value obtained on the image of the stepwedge. The speed of a typical low speed commercial direct positive interms of /E X 10 is 103 and the conventional material has a B of 3.5.

The invention will now be illustrated in and by the following examples:

EXAMPLE I A direct positive emulsion was made in the following manner.To an aqueous solution of gelatin containing 1 mole of potassiumchloride and 6.8 10 mole of rhodium trichloride there was rapidly added1 mole of silver nitrate in an aqueous solution. To the resultingmixture there was added 0.4 mole of potassium bromide in aqueoussolution and the resulting emulsion was then ripened by heating for 10minutes as 160 F. A second portion of 0.6 mole of potassium bromide wasadded and the emulsion was further ripened for 10 minutes at 160 F.

The resulting emulsion was then cooled, washed and redispersed in themanner disclosed in Moede, U.S. Pat. No. 2,772,165, issued Nov. 27,1956.

To 340 grams of 10 percent aqueous gelatin solution there was added0.375 mole of the above silver chlorobromide emulsion. The suspension isstirred, heated to F the pH adjusted to 8.1 with borax. The chemicalfogging agents,

morpholine borane as a control and various azaboranes as shown in thefollowing table, were added and the emulsion was digested at I30 F. for40 minutes. Morpholine borane is disclosed and claimed in assigneespatents noted above as (CH NNB H, shows better aging stability thanmorpholine borane in 5-day oven aging, and (CH ).,N(CHQ NN'B H showsequal or better aging stability on 7-day oven aging. These foggers alsoincreased contrast (less toe) at equal or chemical fogging agents and isused here as a control sample. 5 better D-maX. After digestion, theemulsion was cooled to 95 F the pH was adjusted to 7.0 with acetic acidand a wetting agent, i.e., EXAMPLE H saponin, vvas added- The ViscositlfSuitably adjusted and A direct positive emulsion was made as describedin examlhe emulsion was coated at approximately 700 AgNOa P ple l andcoatings were made of emulsion samples containing square foot on aPhotographlc film PP and drled a the borane compounds shown in the tablebelow for the purventional manner. sensltometric tests were made onfreshly pose demonstrating h ili f h solutions f the coated material andmaterial which had been aged an 7 ounds which are added to theemulsions. All testing was cardays at 120 F. and 62 p rc n r la ivehumid y. h is ried out as in example i. No testing was carried out ofemulgeneral y quiv n to about months normal g gl 5 sions containingmorpholine borane from solutions older than Th n l m m SHIPS wereeXPOSed In the above- 36 hours because of deterioration of the solutionas shown by described sensitometer with Luxometer units through aanalytical tests.

TABLE II Amount Develmole/1.5 Age of Sp Dmaz. Toe oper .AgHal Compoundsolution Ctg. N0

1 1,280 5.67 2.6 II 1groin ZAXIO Morphollne borane 0.001Min ethanolgSfi-hr.

ample 2 777 5. 58 2.7 .ldo- 1.2XI0' (CHahNNBpHp 0.001 M in ethanol. Dayof use. 3 1,040 5.56 1.5 do. 1.2Xl0' d0- 8-day. 4 3,770 6.13 2.3 do1.2)(10 d0 27-day. 5 l, 150 6.58 2.3 d0 1.8)(10' (CH3)4N(CI Day of use.

in ethanol. 6 1,110 5.56 2.4 do 1.8X104 d0 27-day.

N 0TE.See Table I footnote.

2 wedge with 21 steps. The strips were developed in the two (CH).,N(Cl-l l NNB H is more stable than (CH -,NNB H following developersfor 1% minutes at 68 F.

Developer I (High-contrast type) Water to make LG I.

because there is little difference between a fresh (day of use) solutionand 27-day old solution. (Cl-l -,);,NNB,,l-l solution shows littlechange in 8-day age. This is better than morpholine borane (which isonly allowed 36-hr. age) as explained above.

EXAMPLE Ill mole of (CH NNB H, /unit even with a solution that is 26days old. A fresh solution requires a nominal l.2 l()' mole/1.5 molesAgHal (see coating 2 of Ex. I) to give results similar to l.6 l0mole/1.5 moles AgHal of a 26 day old Sensitometric evaluation adducedthe following data: solution.

TABLE I Amount mole] Devel- 1.6 moles Test Sp. Din" Toe oper AgHalCompound age Ctg. No

1 1, 580 5. 73 1. 7 II 2.4X10- Mgrplholine borane 0011- Fresh.

:0 I, 930 5. 74 2. 0 I 2.4Xl0- 2, 590 5.59 2.2 II 2.4)(10- 2, 690 5. 402. 3 I 2.4)(10- 2 2, 070 5. 72 1. 3 II 1.2X10' 2, 5. 76 1. 6 I 1.2)(10-2,260 5.84 1.6 II 1.2XI0- 2, 070 5. 91 I. 5 I 1.2X1U- 3 1, 380 6. 31 1.3 II 2.4Xl0' (CH3)4N(CH )2 Fresh NNBQHH I, 280 6. 06 1.1 I 2.4X10" SameDo. 1,680 5.74 1.9 II 2.4)(10 ..do 7 days. 1, 870 5. 74 1. 9 I 2.4)(10...do Do.

NoTE.Sp.=Speed=l00lEX10-* at density 0.25; D L=Maximum density;

0.25 minus step number at density 0.01. This the greater the contrast.

Toe=Step number at density number gives a measure of contrast in whichthesmaller the number TABLE III Amount Develmole/1.5 oper moles Sp DmaxToe Ex. I AgHal Compound Ctg. N o

1 3, 640 3.07 3 2 II 1.2X10 (CIImNNB H solution aged 26 days.

4, 420 3. 31 3 3 I 1. 2X10' Do. 2 1,630 6. 31 2.8 II 1.6Xl- Do. 2,0006.31 2.4 I 1.6Xl0- Do. 3 777 6.34 2 2 II 2.4X10- Do. 889 6.33 2 1 I2.4X10 D0. Norm-See Table I footnote.

EXAMPLE IV These data show that CSB H S is a more effective compound Adirect positive emulsion was made as described in example I andmorpholine borane and the boron hydride, CsB l-l s, were added aschemical fogging agents as indicated in the following table. All testsand operations were carried out as described in example I. As indicatedin example I, the emulsion containing morpholine borane is used as acontrol. Semitometric evaluation provided the following data:

at low pH than morpholine borane, and maintains D-max more effectivelyin oven aging at low pH.

EXAMPLE VI Direct positive emulsions were made as described in example lto which various boron hydrides were added to demonstrate the widenumber of these compounds which can be used NoTE.See Table I footnote.

These data show the compound, CSB H S, to give greater speed andcontrast (low toe) at approximately equal D-max, and less change on ovenaging than the control compound, morpholine borane, and requiring only0.1 as much fogging agent as the control.

EXAMPLE V A direct positive emulsion was made as described in example Iand morpholine borane as a control and the boron hydride, CsB H S, wasadded to emulsion adjusted to various pH's as indicated in the followingtable. The emulsions were otherwise treated as in example I anddeveloped in Developer as chemical fogging agents for the production ofbleachable fog.

EXAMPLE vn Example 11 was repeated using 2.4 l0 mole/1.5 moles silverhalide, the boron hydride, CsB H, S, as the chemical fogging agent whichwas added from 0.001 molar aqueous solution aged as indicated in thefollowing table. Approximately the same response is obtained regardlessof fogger solution age.

These data show that within the limits of experimental error, 13 weeksold 0.001 Molar CSB H S solution in water ll. produces the same resultas day-of-use solution. With 0.00]

TABLE V Amount mole/1.6 Romelt moles Tost Sp. Din-x. Too pH AglIalFoggor typo Ctg. No.2

1 No dens 3.0 24X10- Morpholine boruno. Fresh. .d0 3.0 24x10 do 5 days.2 3, 900 3.86 1.4 3.0 2:900" CSBQII S Fresh. 3,640 4.18 2.2 3.0 2.4)(10-Same as above fidays. 3 No dens 4.0 24x10- Morpholino b0tan0 Fresh.do... 4.0 24 l0- -do-... 6days. 4 2, 260 5.31 1.8 4.0 2.4)(10- CSBQI'IHSFresh. 1,930 5.08 2.3 4.0 2.4)(10' Same as above 5 days. 5 4, 680 3.512.0 5.0 24X10' Morphollno borano Fresh. 4,680 1.71 3.0 5.0 24)(10- do 5days. 6 497 6.1 1.3 5.0 2.4xl0- CsB H S Fresh. 627 6.1 2.1 5.0 2.4)(10-Same as above fidays.

NoTE.-Seo Table 1 footnote.

molar morpholine borane water solution, it is necessary to EXAMPLE Vllllimit solution age to a maximum of 36 hours in order to obtain Astandard slow speed, fine grain silver chloride emulsion uniformresults. was prepared in a conventional manner. To portions of the TABLEVI Amount Dovelmole/ Sp. Dun. Toe Ex.I AgHal Compound Ctg. No.:

l 2,000 0.23 2.4 I 2.4X10 /UO2CH2 II N=BII1 CH2 CH3 Morpholine boraneControl.

2, 070 a. 27 2. 2 i1 2. 4x10- Same as above.

2 702 6. 1. 4 I 2. 4X10- BnH13S(C2H5)2 752 0.21 1. 6 II 2. 4X10- Same asabove.

3 1. 230 6. 22 2.0 I 2.4)(10 B10 lZ[s(c2H5)g]g 1, 230 6. 2. 5 II 2. 4X10Same as above.

4 827 6.14 1.6 I 2. 4X10 BiuHizCNHa 927 6. 24 1.0 II 2. 4X10- Same asabove.

5 1, 330 5. 22 2. 2 I 2. 4x10- (CH3)4N(CH3)3CNHCB10H13 1, 280 6. 11 2. III 2. 4000- Same as above.

6 1, 870 6.21 2. 2 I 2. 4X10 Morpholine bomne control.

I. 480 6. 21 2. 5 II 2. 4X10 Do.

7 880 6. 23 1. 0 I 2. 4X10 B10H122(C H928 1 027 6. 24 1. 9 II 2. 4X10-Same as above.

a 611 6.18 3.0 1 2.4x10- cnnunliws 603 6.17 2. 0 II 2. 4X10" Same asabove.

0 852 6.18 3.0 I 2.4)(10- (CH3)4NBmHnS 852 0. 17 2. 5 II 2. 4X10- Sameas above.

10 8.080 3. 18 2.5 I 2. 4Xl0 (GH3)3N1IB10III2CII V 7. 780 3. 32 2. 4 II2. 4X10" Same as above.

NOTE. See Table I footnote. 7 i

TABLE VII Cataloging weight mg. AgNOa/ Devel- Fogglng agent Sp. ToeDmlx. sq. ft. Fog oper solution ago Ctg. No

1 4, 290 2.7 4.64 573 .00 I Day-of-use".

4,190 '2. 2 4. 32 573 .00 II Do.

2 5,450 2.4 3.97 503 .00 I 2 days.

5,640 3.4 3. 503 .00 II Do.

3 4,290 2.2 4.83 605 .00 I Sdags.

4 5,640 2.5 4.23 537 .00 I 13 days. 6,030 2.2 3.00 537 .00 II Do.

5 4,290 2. 7 5.02 698 .00 I 15 days. 4,680 2.0 5. 23 698 .00 II Do.

6 5, 450 2. 4 4.00 815 .00 I 13 weeks plus 1 day. 5,640 2. 2 4. 70 81600 II Do.

Nora-See Table I footnotes.

TABLE VIII Fog Fogglng Sp. Dmn. 1X 3X agent Amount Catalog No.:

1 5.11 .00 .05 HCHO oon- 0.1 mole/1.5 Fresh.

trol. moles AgHal. 106 4.65 .00 .04 do "do 2M0.NA.

30 1.70 .00 .00 do "do 7days 0'1.

2 142 4. 71 .00 .00 GsB H S 7X10- mole] Fresh.

1.6 moles AgHal. 142 5.05 .00 .04 Same .do 2 M0. NA. 126 5.17 .00 .00 do"do 7 days OT.-

Natural ng (months). 2 Oven te l mn accelerated aging test considered toequal approximately 10 months natural ago, see Example I).

emulsion there was added formaldehyde and the boron hydride, CsB H S, inthe amounts indicated in Tubl e V lll.

EXAMPLE 1X An emulsion was made as described in example I and morpholineborane as a control and the boron hydride, [(C6H,) .CH.P].B,.H wereadded in an amount of 2.4x 10*? mole per 1.5 moles of silver halide. ThepH of the emulsion containing the control amine borane as adjusted to8.0, and that to which the polyhedral borane was added, was adjusted to7.0. The results are shown in the table.

TABLE IX Develp per, Sp Dmux. Toe x;I Compound Ctg. No

1 2,490 4.16 3.1 I Morp)hollne borane.

2,590 4.25 2.2 II o. 2 4,680 4.57 2.5 I gcuHshcHaPhBnIIu 4, 420 4.63 2.8II ame as above.

NoTE.See Table I footnote.

The boron hydride shows higher speed and maximum density than thecontrol.

EXAMPLE X An emulsion was prepared as described in example I and variousquantities of the boron hydride, CSBBHHS, were added at pH 8.1 to 8.3 togive results as shown in the table.

TABLE X Mole Devel- CsB H S/ paper. 1.5 moles Sp. Durex. Toe Fog x.IAgHal Ctg No 1 12,300 3.49 2.7 .00 I 2.4)(10' 11,900 3.27 2,-7 .00 II2.4X10- 2 2,890 5.97 1.5. .00 I 4X10" 2,990 5.65' 1.6 .00 II 4X10" 31,280 5.81 2.1 .00 I 8X10 1,330 5.64 1.2 .00 II 8X10- 4 965 6.24 1.3 .00I 12X10- 965 6.25 1.8 .00 II 12X10- 5 777 5.60 1.7 .00 I 16X10' 752 5.671.6 .00 II 16x10- 727 5.53 1.5 .00 II 20X10- 7 677 5.63 1.3 .02 I 24X10-725 5.53 1.5 .02 II 24x10- Vslow. VV slow.

7 Nora- 82 1121131 1 footnote.

EXAMPLE XI To demonstrate the usefulness of the boron hydrides aschemical fogging agents under varying low pH conditions an emulsion wasmade described in example I. The boron hydride, CsB H S, was added inthe amounts per 1.5 moles of silver halide and under the pH conditionsindicated in the following table.

TABLE XI Mole Devel- Emul- CSB IInS/ opar, slon 1.5 moles Sp. Dmu. TooFog Ex. 1 p11 AglIul Ctg.No

1 3,640 4.28 3.2 .00 I 3.0 2.4)(10 3,000 3.86 1.4 .00 II 3.0 2.4X10" 22,000 2.50 4.4 .00 I 3.0 12x1o- 2,000 2.67 3.4 .00 II 3.0 12X10- 3 9272.97 2.2 .00 I 3.0 36x10- 965 2.66 1.8 .00 II 3.0 36X10- 4 2,200 4.9 2.7.00 I 4.0 2.4)(10- 2,260 5.3 1.8 .00 II 4.0 2.4X10- 607 5.24 1.8 .00 II4.0 12X10- 6 585 1.05 2.4 .01 I 4.0 30x10- 677 .02 2.0 .00 II 4.0 36 1CH7 479 6.06 1.2 .00 I 5.0- 2.4X10- 497 6.12 1.3 .00 II 5.0 2.4X10- 8 2684.67 .6 .25 I 5.0 12x10- 243 5.77 .3 .00 II 5.0 12X10- 9 252 4.11 .4 .84I 5.0 36X10 293 3.63 .9 .00 II 5.0 36 10- Norm-See Table I footnote.

The preferred concentrations are 2.4X10 to 2.4Xl0' and some of the boronhydrides are considerably more active as chemical fogging agents thanmorpholine borane which is used as a control in many of the aboveexamples and is representative of the amine boranes used as chemicalfogging agents in Bigelow and Burt US. Pat. No. 3,361,564. Many of theboron hydrides act efficiently as chemical fogging agents underconditions of low pH i.e., 3-5 whereas amine boranes require pHsgenerally above 7 and preferably 8.0 to 8.5 to give satisfactoryfogging.

Boron hydrides generally show less loe in the sensitometric curve hencegreater contrast than the amine boranes. The boron hydrides producedirect positive elements having improved fresh and oven aged responsewhich is related to shelfaging over that obtained from amine boranes. Afurther advantage is that the boron hydrides are more stable in thestock solutions used to add them to the silver halide emulsion systemsthan is the case with the amine boranes some of which can not be made upin solution form for more than 36 hours before use.

As is evident from the sensitometn'c data the use of the boron hydridesas chemical fogging agents provide the above advantages without any lossin speed as compared to the amine boranes thus providing extremely fastdirect positive elements. The elements of this invention, as shown inthe examples, may be developed in any standard developing solutioneither of the continuous tone type or the high contrast lithographictype using standard technique. Variations in the developing solutionwill have much the same effect as it would in developing nonreversalemulsions. No preexposure operations or auxiliary processing proceduresare necessary or described in using the novel elements of thisinvention. It is also unnecessary to utilize stain producingnonsensitive dyes or other desensitizing compounds in the emulsion. Theboron hydrides are far superior to formaldehyde, the principle foggingagent of the prior art, because the boranes do not have a hardeningeffect on the gelatin layer.

The emulsions of this invention may be coated on any suitable baseincluding paper and transport film supports. For exterephthalic acid anddimethyl terephthalate with propylene glycol, dimethylene glycol,tetramethylene glycol or cyclohexane-l,4-dimethanol(hexahydro-p-xylenealcohol). The films of Bauer et al., US. Pat. No. 3,059,543 may also beused. The above polyester films are particularly suitable because oftheir dimensionable stability. The emulsions are generally coated on thesupports in quantities to give a coating weight of about 700 mg. AgNO/ftF.

Many boron hydrides other than those listed above can be used aschemical fogging agents for direct positive photographic elements inaccordance with the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A direct-positive, water-permeable colloid-silver halide emulsioncontaining a fogging amount of a boron hydride in which the skeletalframework forms a polyhedron or a fragment thereof and may containheteroskeletal atoms selected from the group C, S, and N.

27 An emulsion according to claim 1, wherein the amount ranges from X10to 24x10 mole per 1.5 moles of silver.

3. An emulsion according to claim 2, wherein the silver halide is silverchloride.

4. An emulsion according toclaim 2, wherein the silver halide is silverchlorobromide.

S. An emulsion according to claim 2, wherein said colloid is gelatin.

6. An emulsion according to claim 2, wherein the boron polyhedralhomonuclear borane of the series ing to claim 2, wherein the boronhydride is a polyhedral heteronuclear borane.

8. A direct-positive photographic element comprising a sheet supportbearing a l defined in claim 2.

ayer of a silver halide emulsion as 9. An element according to claim 8,wherein the silver halide is silver chloride.

BaHu' IO B zHiz 12. An element accord hydride is a polyhedral het 13. Anelement accord hydride is CSB H S.

ing to claim 8, wherein said boron eronuclear borane. ing to claim 8,wherein said boron star-11*

2. An emulsion according to claim 1, wherein the amount ranges from 5 X10 7 to 2.4 X 10 4 mole per 1.5 moles of silver.
 3. An emulsionaccording to claim 2, wherein the silver halide is silver chloride. 4.An emulsion according to claim 2, wherein the silver halide is silverchlorobromide.
 5. An emulsion according to claim 2, wherein said colloidis gelatin.
 6. An emulsion according to claim 2, wherein the boronhydride is a polyhedral homonuclear borane of the series B6H6 2 toB12H12
 2. 7. An emulsion according to claim 2, wherein the boron hydrideis a polyhedral heteronuclear borane.
 8. A direct-positive photographicelement comprising a sheet support bearing a layer of a silver halideemulsion as defined in claim
 2. 9. An element according to claim 8,wherein the silver halide is silver chloride.
 10. An element accordingto claim 8, wherein the silver halide is silver chlorobromide.
 11. Anelement according to claim 8, wherein the boron hydride is a polyhedralhomonuclear borane of the series B6H6 2 to B12H12
 2. 12. An elementaccording to claim 8, wherein said boron hydride is a polyhedralheteronuclear borane.
 13. An element according to claim 8, wherein saidboron hydride is CsB9H12S.